Bacillus thuringiensis (hereinafter B.t.) is capable of producing proteins that accumulate intra-cellularly as crystals. These crystal proteins are toxic to a number of insect larvae. Based on sequence homology and insecticidal specificity, crystal proteins have been categorized into different classes. Best studied are the CryI class of proteins, which are produced as 140 kDa protoxins and are active towards lepidopterans.
To some extent, the mode of action of crystal proteins has been elucidated. After oral uptake, the crystals dissolve in the alkaline environment of the larval midgut. The solubilized proteins are subsequently processed by midgut proteinases to a proteinase-resistant toxic fragment of about 65 kDa, which binds to receptors on epithelial cells of the insect midgut and penetrates the cell membrane. This eventually leads to bursting of the cells and death of the larvae.
The activity spectrum of a particular crystal protein is to a large extent determined by the occurrence of receptors on the midgut epithelial cells of susceptible insects. The activity spectrum is co-determined by the efficiency of solubilization of the crystal protein and its proteolytic activation in vivo.
The importance of the binding of the crystal protein to midgut epithelial receptors is further demonstrated where insects have developed resistance to one of the crystal proteins, such that the binding of crystal proteins to midgut epithelial cells in resistant insects is significantly reduced.
Toxic fragments of crystal proteins are thought to be composed of three distinct structural domains. Domain I, the most N-terminal domain, consists of 7 a-helices. Domain II comprises 3 .beta.-sheets. Domain III, the most C-terminal domain, folds into a .beta.-sandwich. If projected on CryI sequences, domain I runs from about amino acid residues 28 to 260, domain II from about 260 to 460, and domain III from about 460 to 600.